The present invention relates to a method and a device for producing preforms for molding an advantageous base and shoulder geometry for the subsequent blow-molding process in first heat or second heat.
Preforms are primary products within PET bottle production which are either stretch-blow-molded to form PET bottles in the still hot state immediately after production in an integrated method (single-stage process) or are stretch-blow-molded to form PET bottles in a second process stage after cooling down from the production process in a two-stage process.
For the customary production of preforms described in this invention, polymer raw material is plasticized and subsequently forced at high pressure into a single- or multi-cavity mold.
This produces preforms according to FIG. 1, which geometrically consist substantially of a neck and shaft region and a domed base end and are hollow on the inside due to the insertion of a core in the mold. The neck region is shaped in such a way that it may be configured for example so as to be re-closable with a screw cap. The neck region does not undergo any further change during the blow-molding process, however. By contrast, the shaft region and the domed base end are inflated at elevated temperatures to form hollow bodies, whereby the polymer is stretched and at the same time considerably solidified. Therefore, in conjunction with the core geometry, the preform regions to be deformed are geometrically responsible for the bottle quality that is subsequently obtained.
In the single-stage process illustrated here by way of example, an injection mold and a subsequent blow mold are conventionally used. Since the injection-molding operation lasts much longer than the blow-molding operation, there are system solutions in which the number of injection-molding cavities is a multiple of the blow-molding cavities.
The injection-molded preform, the outer skin of which is in direct contact with the intensively cooled steel of the mold, consequently solidifies quickly there and can thus be demolded without any damage and without mechanical deformation. By virtue of the considerable residual heat inside the preform wall, which results in re-heating and associated re-softening without any further use of thermal energy in the single-stage process, the preform can be inflated to form the hollow body in the next production step. However, it is very difficult to give the preform a thermal profile that is optimal for stretch blow-molding—unless the single-stage injection stretch-blow-molding machine is equipped with an additional station with IR heaters or a conditioning station, which can have an influence on the thermal profile. However, even then, the thermal profile—particularly in the base and shoulder region—is still not yet optimal. On the finished hollow body, the base and the shoulder frequently have excessive material accumulations.
In the two-stage process, although the preform is reheated in a targeted manner, it is not possible, using the current state of the art, to supply the base and shoulder part with sufficient thermal energy for an optimal blow-molding result. This can be explained primarily by the position of the shoulder and base with respect to the IR heaters.
The preform, as illustrated in FIG. 1, corresponds to the current state of the art, in which it is inevitable that the wall thicknesses of the preform have similar wall thicknesses, particularly in the region of the domed base end and the shaft. If the material sets prematurely on account of thinner wall thicknesses in the gating region, shrinking in the cooling phase on account of follow-up pressure on the melt cannot be avoided, with an effect on the entire preform including the neck region, this consequently leading to undesired sink marks in critical regions of the preform. For this reason, the preform geometry, as shown in FIG. 2 and the advantages of which will be explained below, cannot be produced by the known injection-molding process. The wall thickness is much thinner in the region of the shoulder, but especially in the domed preform end located in the vicinity of the gate, than in the further progression of the preform shaft, and consequently premature setting of this thin region is unavoidable.